1. Field of the Invention
The present invention relates to a method of adjusting image forming positions, which can be applied to an image forming apparatus so arranged that images can be simultaneously formed on a plurality of sheets of recording paper fed in parallel. The present invention also relates to an image forming apparatus having an improved arrangement that enables an adjustment of image forming positions on a plurality of sheets of recording paper fed in parallel. Examples of the image forming apparatus are a copying machine and a laser printer.
2. Description of the Related Art
Laser beam printers for recording images on recording paper in accordance with an electrophotographic process have been widely used. In such laser beam printers, image formation is conducted in the following manner: a photoconductor electrostatically charged evenly throughout its surface is scanned by a laser beam to form an electrostatic latent image corresponding to an image to be formed. The electrostatic latent image is developed into a toner image, and thereafter, the toner image is transferred and fixed to the recording paper.
For example, in a laser beam printer of which the maximum print size is equivalent to a size of Line B, No. 4 of the Japan Industrial Standard (referred to as "B4-size" hereinafter), a photoconductor which can cover a short side of a B4-size sheet is provided. When a photoconductor shaped in a right circular cylinder is used, it would be designed so that a length of its extension along an axial direction is longer than the short side of the B4-size sheet. The "B4-size" referred to herein is one of typical patterns of paper sheet dimensions prescribed according to the Japan Industrial Standard, and measures 257 (mm).times.364 (mm).
Such printers can perform printing in sheets considerably smaller than B4-size, such as post cards and envelopes. However, most parts of the photoconductor are not used in recording images in such small size sheets. Thus, for the most parts of the photoconductor, electrostatic charge and exposure to the laser beam are uselessly conducted. Thus, there arises a problem that fatigue of the photoconductor proceeds regardless of the size of the recording paper.
On recent years, an apparatus which can record images in two sheets of recording paper in parallel for small recording paper, such as envelopes, has been developed. In such an apparatus, two sheets of recording paper are fed to the photoconductor in parallel. Then, images for the two sheets are written onto a surface of a photoconductor by the laser beam.
More specifically, as depicted in a schematic view of FIG. 7, two envelopes 1 and 2 are fed in parallel simultaneously to a photoconductor 10 shaped in a right circular cylinder. Laser beam 5 emitted by a laser light source (not shown) scans the photoconductor 10 along its elongated extension. The photoconductor 10 is rotated about its axial line at a fixed speed.
The laser beam 5 is modulated suitable to images to be recorded on the envelopes 1 and 2. Thus, in a region corresponding to the envelope 1, the laser beam 5 modulated suitable to the image to be recorded on the envelope 1 reaches the surface of the photoconductor 10. Similarly, the laser beam 5, when applied in a region corresponding to the envelope 2, is modulated suitable to the image to be recorded on the envelope 2.
A beam detector 7 is provided in a specified position on an upstream side from the photoconductor 10 along a scanning direction 6 of the laser beam 5. The process for modulating the laser beam 5 corresponding to the image to be formed is started at a specified timing after the beam detector 7 detects the laser beam 5 and outputs a beam detecting signal expressing the detection. This ensures that an electrostatic latent image corresponding to the desired image is written at a predetermined position of the photoconductor 10.
Specifically, as shown in FIG. 8(a), based upon the beam detecting signal output by the beam detector 7, a horizontal synchronizing signal, which is utilized as a reference for timing of data output, is produced. After a specified period of time .DELTA.T has elapsed from output of the horizontal synchronizing signal, a video signal is output to modulate the laser beam 5 suitable to the image data (depicted in a shadowed part in FIG. 8(b)).
Around the photoconductor 10, the following devices are arranged an electrostatic charger for electrostatically charging the photoconductor 10 uniformly throughout its surface before exposure to the laser beam 5, a developing device for developing an electrostatic latent image formed by exposure to the laser beam 5 into a toner image, a transferer for transferring the toner image to the envelopes 1 and 2, and a cleaning device for removing residual toner on the surface of the photoconductor 10 after the transfer. These components work together to record images on the envelopes 1 and 2.
In such an arrangement, image recording can be conducted on the envelopes 1 and 2 in parallel, and therefore, the photoconductor 10 can be effectively used. Thus, when image formation is performed onto small recording paper, the photoconductor 10 can be less fatigued relative to the number of sheets processed. Moreover, since image recording is conducted on two sheets of the recording paper in parallel, image recording can be performed at high speed.
Meanwhile, when it is desirable to form images accurately registrated with a pair of the envelopes 1 and 2, it is essential to make a mutual adjustment between a paper feed mechanism for feeding the envelopes 1 and 2, and an image forming unit including the photoconductor 10 and the like. Specifically, even among printers having the same constitution, it is inevitable that there will be variations of 1 mm to 2 mm in relationship between positions where the envelopes 1 and 2 are fed to and come in contact with the photoconductor 10 and positions on the photoconductor 10 where images are written by the laser beam 5. Hence, adjustment between the paper feed mechanism and the image forming unit is required in the manufacturing line for each printer.
A technique of such adjustment will be described with reference to FIGS. 9(a) and 9(b). As shown in FIG. 9(a), for example, it is assumed that, on the surface of the photoconductor 10, a writing start position WS1 is deviated by .DELTA.W1 from the position where the envelope 1 is in contact with the photoconductor 10 to a downstream side of the scanning direction 6. The writing start position WS1 is on a scanning line 15, and image writing by the laser beam 5 is started from the position. Such deviation in position can be recognized if the image recording onto the envelopes 1 and 2 is performed on trial.
In such a case, a period of time .DELTA.T from output of a horizontal synchronizing signal until output of a video signal (see FIGS. 8(a) and (b)) is shortened. In this way, as shown in FIG. 9(b), since the writing start position WS1 is deviated to an upstream side along the scanning direction 6, the writing start position WS1 and the position where the envelope 1 is in contact with the photoconductor 10 can be registrated. Eventually, the image recording on the envelope 1 can be performed at good positional accuracy.
If it is desirable to deviate the writing start position WS1 to the downstream side along the scanning direction 6, there is a need to extend a period of time .DELTA.T from the output of the horizontal synchronizing signal until the output of the video signal to modulate the laser beam 5.
With the adjustment as mentioned above in the manufacturing line before shipment of products, the image recording on the envelopes 1 and 2 is to be conducted well when the products are delivered to users.
However, with the adjustment technique as stated above, as the writing start position WS1 is deviated, a writing start position WS2 for the envelope 2 is also deviated by the equivalent distance along the scanning direction 6. Hence, when a distance D between the envelope 1 and the envelope 2 varies among manufactured printers, variations in the distance D cannot be absorbed by the adjustment as mentioned above.
For example, as shown in FIG. 9(b), there occurs a case where the adjustment as stated above causes the writing start position WS2 for the envelope 2 to be deviated by .DELTA.W2 in spite of accurate positioning as to the writing start position WS1. The adjusting technique as stated above cannot vary the writing start positions WS1 and WS2 for the envelopes 1 and 2 separately, and eventually, the deviation .DELTA.W2 is unavoidable.
In practice, it is empirically found that the distance D between the envelope 1 and the envelope 2 varies 1 mm to 2 mm among the yielded printers. More precise arrangement of the paper feed mechanism might suppress the variations in the distance D, but it is impractical because of excessive cost increase.